The structure of skin can be visualized best in terms of its basic division into two anatomically distinct layers: the epidermis and the dermis. The epidermis is the only portion of the skin normally visible. It is a multicellular layer and gives rise to all the glands and appendages of the skin. The dermis forms the bulk of the living skin and fundamentally has a fibrous structure.
The epidermis is composed of two principal layers: the stratum malphighii (the wet living epidermis) and the stratum corneum or horny layer (the dried, keratinized, "dead" outermost layer). The epidermis contains two distinct cell types: keratinocytes which are the primary structural cells comprising 90-95% of the epidermis and melanocytes which are the melanin synthesizing cells, comprising 5-10% of the cells normally found in the basal layer of the malphighian zone.
The color of human skin is due to presence of melanin pigment within the skin, which pigment is usually in the form of discrete granules called melanosomes. The latter are generated in the viable layer of epidermis by pigment cells (melanocytes). The skin color is genetically controlled and people of different races exhibit characteristic features of pigmentation both in their intensity and chromatic tone. The depth of color in human skin is a function of the quantity of the melanin pigment present, its chemical characteristics and distribution. The presence of melanin in the skin provides a marked photoprotective role, i.e., it acts as an oxygen radical scavenger and as a sunscreen. Black and darkly pigmented people are far less susceptible to chronic actinic damage from ultraviolet light than are fair skin individuals.
It is well known that individuals can augment their skin color by tanning. Exposure to ultraviolet radiation, whether from sunlight or artificial sources such as sunlamps, results in formation of additional pigment in skin. The tanned skin is almost universally regarded as a positive aesthetic attribute, and it is highly desired as a sign of beauty and health. A high proportion of fairskinned (Caucasian) individuals have this desire.
Unfortunately, this desire for tanning subjects skin to the damaging effects of ultraviolet radiation. The most readily perceived and acutely felt effects of ultraviolet exposure is the erythema (skin reddening and burning) but this is only a prelude to actinic elastosis (sailor's skin) and possibly even squamous cell carcinoma.
Ultraviolet light that reaches the earth's surface is divided generally into a higher energy region (UVB) having a wavelength band of 2900 to 3400 Angstroms, and a lower energy region (UVA) having a wavelength band of 3400 to 4000 Angstroms. Radiation in the UVB range usually causes the skin to burn as well as tan. A satisfactory level of tanning and somewhat less burning can be attained by prolonged exposure to ultraviolet light in the UVA range.
Some protection against sunburn and other ultraviolet radiation exposure related problems is provided by sunscreens, i.e., materials which are effective in absorbing ultraviolet radiation in the range of 290-320 nm. While application of these may prevent skin burn, it also retards or inhibits tanning. Here, indeed, lies the dilemma of tanning. The melanin pigment itself is one of the best protecting agents against the harmful effects of ultraviolet radiation but to generate it by tanning, one pays the price of sun-induced skin damage.
To satisfy the aesthetic longing for tan skin without the necessity of exposing oneself to the harmful effect of prolonged sun exposure, a number of types of skin coloring products have been developed, two of which are on the market.
The first type of products consists of skin gels and lotions which contain dyes and/or pigments that when spread on skin surface mimic the tanning effect. However, while the color characteristics (natural look, tone, etc.) obtained with these products are satisfactory, the surface deposits are only of temporary nature, they stain clothing and undergo ready rub-off.
The second type of products, of a more durable character, are those that are based on materials that interact with skin such as iodine, potassium permanganate and dihydroxyacetone. Of these, only the dihydroxyacetone has gained some limited commercial importance, but even in this case, the color effects obtained are not aesthetically satisfying.
In addition, a type of product based on the use of a melanin precursor in combination with an enzyme catalyst and frequently an activator have also been described in the literature. U.S. Pat. No. 4,609,544 discloses tanning by applying a composition comprising a melanin like dye, a peroxidase enzyme and hydrogen peroxide in a cosmetic base suitable for topical application to the skin. U.S Pat. No. 4,515,773 discloses a skin tanning composition containing a melanin precursor and a tyrosinase enzyme in a cosmetic base.
However, these prior art compositions require the use of enzymes and oxidative agents in combination with the melanin precursors and appear to deposit the melanin-like materials only on the exterior of the skin. As the melanin molecule is incapable of penetrating the skin, only superficial external tanning results, which is readily removed by rinsing with water or rubbing with a towel. There is truly no commercially available product that will mimic skin tan with respect to its durability, aesthetics of color and photo-protection. A need thus clearly exists for a preparation to produce a durable and natural appearance of tan combined with protection against potentially harmful sun radiation.
Accordingly, it is an object of this invention to provide an improved sunscreen capable of reducing burning and promoting tanning.